Known mufflers for internal combustion engines have a valve capable of closing and opening an open end of by-pass inner pipe. When the revolving speed of internal combustion engine is high, the open end of the muffler is opened to reduce exhaust resistance, thereby mitigating back pressure. When the revolving speed is low, the open end is closed to increase the muffling effect.
For instance, a Japanese Unexamined Patent Publication No. 5-98930 proposes an arrangement shown in FIG. 6. In that figure, a valve 103 is fixed at the foot of an open end 101a of a by-pass inner pipe 101, and is pivotable with respect to a connection plate 104 serving as a fulcrum. The connection plate 104 is made of flexible metal and has a bimetal 105 attached to its outer surface. In assembly, the valve 103 is positioned at the position indicated by the bold line in FIG. 6.
In this arrangement, when the revolving speed of internal combustion engine is high and the exhaust gas becomes hot, the bimetal 105 is heated and bends as a result of the temperature change. Responsively, the valve 103 opens wide and reaches the position indicated by a dot-dash line in FIG. 6, thereby reducing back pressure. On the other hand, when the revolving speed is low and the exhaust gas is of a lower temperature, the valve 103 reaches the position indicated by a broken line in FIG. 6 and closes the open end 101a, thereby providing an increased muffling effect. In such arrangement, even if the temperature of the exhaust gas is relatively low, it is still higher compared to the temperature during the assembly of the valve 103. In order to allow for the bending degree of the bimetal 105 which varies according to temperatures, in the arrangement the open end 101a projects forward in a predetermined angle.
Meanwhile, it has also been proposed to utilize means other than temperature in controlling the valve.
For instance, a Japanese Unexamined Utility Model Publication No. 2-3009 controls closing and opening of an open end by means of pressure. Specifically, as shown in FIG. 7(a), the prior art suggests a muffler having a housing 116 the inner space of which is divided into three chambers. Into separate expansion chambers 112, 114 are inlet pipe 118 and an outlet pipe 120 inserted. The expansion chambers 112, 114 communicate with one another via two parallel inner pipes 126, 128, one 128 (hereinafter referred to as "by-pass inner pipe") of which has an open end 128a, at the lower reaches of the exhaust gas stream, which is provided with a valve 132 for a valve mechanism 130.
In such a muffler, the by-pass inner pipe 128 opens, at the upper reaches of exhaust gas stream, into the first expansion chamber 112. When the pressure inside the first expansion chamber 112 is equal to or below a predetermined value, the urging force from a spring 142, shown in FIG. 7(b), of the valve mechanism 130 and the pressure inside the second expansion chamber 114 are together acting on the valve 132 to keep the open end 128a closed. On the other hand, when the pressure becomes equal to or above a predetermined value, the valve 132 resists the urging force from the spring 142 of the valve mechanism 130 and other pressures, and retracts away from the open end 128a, thereby opening the open end 128a. Thus, when the revolving speed is low and so is the pressure, the open end 128a is closed by the urging force from the spring 142 and other pressures, and an enhanced muffling effect is provided. On the other hand, when the revolving speed is high and the pressure is high enough to open the open end 128a, further rise of the back pressure is prevented. Since the closing and opening of the open end 128a by the valve 132 occurs in a quick response to the shift of the exhaust gas pressure, it is enabled to control the valve 132 appropriately in accordance with the revolving speed of internal combustion engine.
However, as previously described, the former type of muffler utilizes the property of the bimetal 105 which bends in a varied degree according to temperatures. Since this method has a problematic time lag intrinsic to the bimetal 105 in responding to the shift of the exhaust gas temperature and reaching a critical temperature, it is quite difficult to have the revolving speed of internal combustion engine appropriately reflected upon the control of the valve 103 to be closed or opened, and an inappropriate control is resulted.
On the other hand, the latter type of muffler requires a number of components other than the valve 132 and the spring 142 urging the valve 132 against the open end 128a. Such components include, as shown in FIG. 7(b), a shaft 140 to support the valve 132, and a piston 144 and cylinder 146 which are connected to the shaft 140. The numerous components add to cost and because, as shown in FIG. 7(a), they need to be assembled to the exterior of the housing 116, the assembly is laborious and needs protective measures against external factors, such as smash of stones and salt injury.